1. Field of the Invention
The invention relates to a method for controlling a hydraulic brake system of a motor vehicle equipped with a braking assistance function, and to a hydraulic brake system of this type.
2. Description of Related Art
Brake systems of motor vehicles are generally hydraulic brake systems. In this context, a master brake cylinder generates a pressure by shifting a pressure piston, connected directly or indirectly to a brake pedal, in a pressure chamber filled with brake fluid. The pressure passed on to at least one wheel brake cylinder via hydraulic lines. The wheel brake cylinder moves brake linings, bearing, for example, against brake shoes owing to the pressure of the brake fluid, into abutment with a brake drum or a brake disk. In this way, the braking force acting on the wheel connected to the brake drum or brake disk can be metered by activation of the master brake cylinder. The master brake cylinder is generally embodied as a tandem master brake cylinder (tandem master cylinder, TMC) that has two pressure chambers for applying pressure to two independent brake circuits. The two brake circuits can each act on all the wheels of the motor vehicle, but can also be distributed, for example, between the individual wheels in such a way that one of the brake circuits acts on the front wheels and the other on the rear wheels.
The master brake cylinder can be activated by the driver of the motor vehicle using the brake pedal, wherein to boost the braking force a brake booster can be connected to the pressure piston of the master brake cylinder. The brake booster generates, as a function of the activation of the brake pedal, an additional force that acts in the activation direction of the brake pedal, because of which the pressure piston is moved with increased force in the activation direction. The brake booster frequently generates the additional force pneumatically, in particular by means of under pressure, but can also generate said additional force electromechanically, in particular by means of an electric motor that shifts a valve body in the activation direction by means of an actuator gear mechanism.
If the brake pedal is activated and an increased pressure is generated in the pressure chamber or chambers of the master brake cylinder by means of the pressure piston, brake fluid is expelled from the pressure chamber or chambers in order to activate the wheel brake cylinder or cylinders. In the case of a motor vehicle equipped with a braking assistance function, i.e. with an automatic braking function, also referred to as a “braking assistant”, an additional increase in brake pressure can take place automatically in certain situations in order to increase the braking forces applied to the wheels, and to shorten the braking distance of the motor vehicle.
Such a situation in which the braking assistance function intervenes may be, for example, a very rapid activation of the brake pedal by the driver during which, however, a brake pressure that is sufficient for full braking is not reached. Owing to the high speed of the activation of the brake pedal, it is to be assumed in this case that the driver will initiate full braking, but does not apply the activation force necessary to bring it about. As a result of the intervention of the braking assistance function, additional brake pressure is generated which is sufficient for full braking, in particular for activation of an anti-lock brake system (ABS).
Another situation in which the “braking assistant” intervenes can occur if the brake booster has reached its maximum force but a still greater degree of deceleration and therefore even higher brake pressure is required. In this case, an increased brake pressure can be generated by intervention of the braking assistance function without the brake pedal feeling hard.
A further situation in which intervention by the braking assistance function is advantageous can occur if the driver of the motor vehicle brakes with a considerable braking deceleration, for example with 3 m/s2, and an automatic detection system detects an obstacle in front of the motor vehicle in the direction of travel, but the braking deceleration selected by the driver may not be sufficient to avoid a collision with the obstacle. In this case, the “braking assistant” can automatically generate a relatively high brake pressure and therefore bring about a greater braking deceleration, with the result that the collision can be avoided or at least attenuated.
A pressure-generating device of an electronic stability control system (Electronic Stability Control, ESC) can bring about an increase in the brake pressure during the intervention of the braking assistance function. If this pressure-generating device is activated, that is to say, for example, a fluid pump for generating pressure operates, this generally brings about a reduction in the pressure in the master brake cylinder, resulting in a reduction in the force applied to the brake pedal by the driver or even in the brake pedal dropping by several centimeters. This behavior of the brake system impedes the sensing of the braking behavior desired by the driver; in particular, it is difficult to determine whether the driver wishes to reduce the braking force or even to end the braking process. Likewise, it is difficult to determine whether the driver wishes even higher braking deceleration. As a result, it is made more difficult, or even impossible, for the driver to control the braking process by means of the force applied to the brake pedal.